As it is well known, a variable cam phaser is used to change the cam lobe (valve lift event) timing to crankshaft timing while the engine is running, based on the parameters of the engine. Thereby, optimum values for fuel consumption and exhaust emissions can be obtained in different areas of the engine's operating characteristics. An elegant manner of varying the valve timing is realized by rotating the camshaft relative to its driving member, typically a sprocket wheel or pulley connected to the crankshaft via a chain, respectively a toothed belt.
Various types of cam phasers capable of achieving this exist. Conventional cam phasers employ hydraulic actuators using high-pressure oil to enable relative angular displacement between drive and driven members. Unfortunately, hydraulic systems have difficulty operating at extremes of temperature, in particular during engine start up when the oil is cold, due to temperature related viscosity changes of the oil.
To avoid such problems, more recent designs of cam phasers employ electric actuators in a configuration allowing their mounting at one end of the camshaft. The cam phaser typically has coaxial input and output members, the input member being coupled to the engine's sprocket wheel to act as drive member, while the output member is coupled to the camshaft. In practice, the sprocket wheel often has an extension with a toothed portion that meshes with a pinion forming the cam phaser's input member and the output member is screwed directly onto the camshaft end. An electrically actuated adjusting mechanism drivingly connects the input and output members enabling selective angular adjustment of the output shaft while maintaining driving engagement between the input and output members. The adjusting mechanism may typically comprise a gearbox arrangement in the form of a planetary gear system or a harmonic drive.
Hence, electrically driven cam phasers comprise a relatively complex gearbox/reductor arrangement that must be manufactured with great care and precision to avoid locking and breaking of the gearings. Furthermore, the cam phaser, which is directly loaded by the sprocket wheel, is subject to important mechanical stresses and this is a cause of rapid wear and/or breaking of many cam phasers.
U.S. Pat. No. 6,328,006 e.g. describes a cam phaser with harmonic drive. U.S. Pat. No. 6,981,478 on the other hand describes a cam phaser with planetary gearing system.